In the electrolysis of an acidic aqueous solution, such as an electrolytic collection of a metal, electroplating, etc., a lead electrode was mainly used as the anode. Recently, in place of the lead electrode, an insoluble metal electrode, prepared by coating an electrode material solution containing a platinum group metal on the surface of a corrosion resistant valve metal, such as titanium, etc., and thermally decomposing the resulting coating in an oxidizing atmosphere at a temperature of from 400.degree. C. to 600.degree. C. to form an oxide coating, has been used. The utilization of such an insoluble metal electrode as an electrolytic electrode in a large-scale high-current density application, such as for high-speed zinc plating, copper foil production, etc., has recently increased because of the durability, the dimensional stability, and the ease with which the insoluble metal electrode can be shaped.
For example, in high-speed zinc plating, a large electrode having one anode area of about 2 m.sup.2 is sometimes used, and when the maximum current density is 20 kA/m.sup.2, an electric current of about 40 kA is passed through one anode. Also, in the anode for producing electrolytic copper foils, one anode area is about 4 m.sup.2 and the electric current sometimes is about 30 kA. Also, in electrolysis, a non-uniform electric current distribution causes products to have extremely poor quality, such that it has particularly been required to make the electric current distribution uniform.
Thus, even where a metal having a good electro-conductivity, such as titanium, is used as the electrode substrate in order to pass a large electric current, it is necessary to ensure that the thickness of the electrode substrate is 10 mm or more, and, as the case may be, an electrode substrate having a thickness of 40 mm or more is used.
On the other hand, coating an electrode material on the electrode substrate is generally carried out by thermally decomposing the coating of the electrode material contained in a liquid. Also, in the case of an electrode substrate having a large thickness for passing a large electric current, from 30 minutes to one hour is required to raise the temperature to the thermal decomposition temperature of from 450.degree. C. to 600.degree. C., and after carrying out the thermal decomposition for 10 to 15 minutes, a time of at least 2 hours is required for maintaining the temperature and allowing it to cool. Furthermore, for obtaining a desired coating thickness on the electrode material, the coating and thermal decomposition operation described above is carried out repeatedly from 10 to several tens of times, and sometimes coating the electrode material may take one to two weeks or longer.
To overcome these problems, it has been proposed to use an electrode structure wherein the electrode substrate for supplying the current to the electrode and for supporting the electrode and the electrode portion of forming the coated layer of the electrode material are separately prepared and the electrode is fixed to the electrode substrate by screws or stud bolts which are fixed to the electrode.
However, even in this method, since it is required to form screws in the electrode or to form other connecting means thereto, the thickness of the electrode is required to be from about 3 to 10 mm.
Only the method of heating such an electrode is far easier when compared with the conventional method of carrying out the heat treatment of the entire electrode structure, but it is not capable of shortening the heating and cooling times. Also, since various fixing means for fixing the electrode substrate are formed on the electrode plate, the area around the fixing means has a slightly different thermal environment from that of other portions, and as a result, a problem results in that the characteristics of the electrode are changed. Moreover, since in the conventional electrode structure, fixing the electrode to the electrode substrate is carried out at the back surface of the electrode, it is difficult to exchange the electrode when the electrode is fixed to the electrolytic apparatus.
Thus, the inventors previously proposed a method of fixing a thin electrode to the surface of an electrode plate by welding Or screws in JP-A-5-171486 and JP-A-5-202498 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
By this method, the electrode can be exchanged when fixing the electrode structure to the electrolytic cell, and it becomes easy to form the electrode Coating, and as a result, the method can be used without any problems until the current density is about 100 A/m.sup.2. However, since the supply of the electric current from the electrode substrate to the electrode plate is only carried out at the fixed screw portion or the welded portion, the electric current is concentrated at these portions. Thus, for passing an electric current having a large current density, it is required to mainly increase the number of the fixed portions or to increase the thickness of the electrode.